If it has a panel on the back, take it off so you can see both main pulley & also pulley on motor. They don't normally just fly off for no reason, so check the main pulley is not loose or wobbly. Grip it with 2 hands and try to wobble it. If you can then you either need to tighten it up or get a new pulley because belt will just come off again. If pulley ok then put belt around motor pulley and then feed it slowly on to main pulley as you rotate the pulley. If no back panel then you take off the top and do it same way but more difficult because you can't see what is happening.
Lincoln Appliance NZ, November 2014